Bathing appliances such as hot tubs, swimming pools, shower units and hydromassage bath fixtures employ a means to control the flow of electrical energy to the pump motor, air blower, or other electrical loads associated with their use. The prior art means of providing this control may be very simple, for example a wall mounted switch or mechanical timer may be utilised to turn the circulation pump on and off.
An issue with bathing appliances, swimming pools and the like is cooling of the bathing water during operation. This may be caused by direct evaporation to cool night air in the case of a swimming pool or through the introduction of ambient air used to operate a bathing system hydrotherapy jets. The human body is keenly tuned to a very narrow “thermal comfort zone” and straying outside of this area by even a few degrees causes discomfort. For example, a person submerged in a hot-tub would find the water temperature cool below 38 degrees Celsius, while 41 degrees would be uncomfortably hot for most people.
It is obvious that water temperature requires careful regulation and that additional energy is required to provide makeup heat to offset natural cooling for the reasons cited above. It is common to provide this makeup heat by a hot water supply tap or by utilizing an integrated electric, gas-fired or other auxiliary heating device.
During operation, the electric motor of the recirculation pump has been found to become quite hot, requiring the use of cooling fans mounted as an integral part of the motor design. In addition, motors and pumps are generally mounted in enclosed areas, wherein the recirculation of cooling air further increases electric motor temperature. Manufacturers provide absolute operating temperature limits for their motor/pump devices to ensure equipment is not thermally damaged. It is well known to those skilled in the art that safety and regulatory agencies are very concerned about thermal damage to electric motors and that design and operating standards have been developed by many countries throughout the world.
In the past, it has been known to wrap a tubular coil around the pump motor and conduct bypass water through the tubular arrangement to extract heat for motor cooling and supplementary water heating. Improvements in this design have included “snap-on” plastic heat recovery jackets and oil-immersed motors with heat bypass tubes integrally mounted within the oil chamber.
Although each of the above prior art designs works more or less effectively than integrated fans to provide supplementary heat and lower motor operating temperature, they do not provide any control over the amount of heating power or delivered heat energy. For example, a given heat recovery system may provide 200 watts of power where 800 watts is required to maintain thermal equilibrium of the water. In such an example, supplementary heating would be required, rendering the heat recovery system of little heating capacity value.
Likewise, a heat recovery system may provide more heat to the water than is lost. Such a situation would result in water temperatures rising over time due to excessive heat energy input (heating power multiplied by time operated), perhaps beyond the safe operating limits determined by regulatory agencies and safety organizations. For example, a hot-tub which is certified to Underwriters Laboratories Inc. applicable standards must not exceed 41 degrees Celsius during normal operation.
A typical bathing appliance, hot tub or swimming pool pump utilises an alternating current, asynchronous electric motor, most commonly known as an induction motor. Induction motor design is well know to those skilled in the art. Motors of this configuration operate at a nominally fixed speed of rotation dependent on the number of magnetic poles in the motor and the frequency of the applied voltage. Motor designers optimize the motor design and fix the “voltage to frequency ratio” (hereinafter “V/F ratio”) to provide maximum mechanical output while minimizing energy losses which result in the unwanted by-product of heat.
A typical motor connected to the North American supply mains circuit will have a V/F ratio of 2 (120 V÷60 Hz). Should the applied voltage to the motor be lowered while the frequency remains constant, the resulting V/F ratio will also lower resulting in high operating current, lower efficiency and additional heat output.
At some critical point on the V/F ratio curve, in combination with ambient air temperature and mechanical load, motor internal heating will exceed the ability of the internal fan to remove this heat and thermal runaway will occur. The motor's internal protective device will trip and the motor will cease to operate until it cools below a preset temperature.
If an effective heat recovery apparatus is incorporated in or upon the motor assembly, excess waste heat may be effectively removed and transferred to the bathing water.